1. Field of the Invention
The present invention relates to a system for the automated measurement of skin perfusion pressure of a local or regional body site. More particularly the invention relates to a system that includes a measuring means for measuring capillary blood flow and placement means for assuring reproducibility, pressure means for applying pressure to a tissue site having capillary blood flow and means for measuring the applied pressure, and means for determining the relationship therebetween that detects and rejects motion artifact and indicates an SPP value.
2. Description of the Related Art
Skin perfusion pressure measurements are taken to determine whether local blood flow, i.e. capillary perfusion, of a local or regional body site having an ulcer or wound is sufficient to support wound healing. The accurate measurement of this parameter, therefore, is critical to physicians who treat patients suffering from open surface wounds resulting from complications from diabetes, pressure ulcers, burns, accidents, and the like.
Traditionally, skin or surface perfusion pressure is measured utilizing a surface or skin perfusion pressure-monitoring device coupled to a laser Doppler or other type of optical sensor. For example, U.S. Pat. No. 6,178,342 to Borgos et al. discloses a surface perfusion pressure instrumentation used in conjunction with a laser Doppler probe that measures the “amount” of moving blood contained within a microvascular observation volume in percent tissue hematocrit. This measurement is taken as a function of applied pressure. The laser Doppler optical probe defines an observation volume in the skin near the surface of the patient and a pressure cuff is used to manually apply pressure to the limb near the optical probe.
The laser Doppler sensor is placed against the skin under a pneumatic cuff that is secured to the affected limb, i.e. toe, ankle, arm, leg, etc. A user using an inflation bulb manually inflates the pneumatic cuff. The inflation pressure must be sufficiently high to stop local blood flow at the site of the optical probe. A display instrument is coupled to the optical probe typically via a fiber optic cable, and to the inflation bulb through a tube. Deflation starts and the optical probe monitors the number of moving red blood cells moving into or out of the observation volume without regard to velocity. The number of moving red blood cells detected within the control volume is expressed as a percent and displayed on the display monitor. This value is shown as both a numeric value and a bar graph on the Y-axis. The instrument also measures the pressure within the cuff and displays the applied cuff pressure in millimeters of mercury on the X-axis of the display. A moving bar chart along the X-axis shows the operator which cuff pressure is currently being measured. As pressure is slowly manually released, an indicator of blood flow return is provided in bar chart form. While a technician conducts the test, a physician interprets the data displayed on the display monitor.
Therefore, a significant problem with the use of skin perfusion pressure instruments described by Borgos et al. is that reliable, reproducible measurements are heavily dependent on operator/technician skill and the skill of the physician who interprets the surface perfusion pressure measurement. Another problem associated with manual deflation is that it is sensitive to motion artifact caused by the operator or patient (e.g. patient movement, pressure tubing movement, or sensor movement). In addition, motion artifact may result from patient movement, involuntary muscular movement, operator intervention, and other causes affecting the reading of skin perfusion pressure. If a patient moves the limb to which the sensor/pneumatic cuff is attached, the physician who makes the determination of the pressure at which flow returns might very easily err by reading “motion artifact” as the surface perfusion pressure measurement. When a skin perfusion pressure test is conducted on a sick patient, the physician is already expecting a low value for the surface perfusion pressure measurement. Consequently, given an occurrence of “motion artifact” the physician may interpret it as a skin perfusion pressure reading that is artificially higher than the actual skin perfusion pressure measurement.
For example, illustrated in FIG. 2A is a display from a prior art monitor. As can be seen, measured perfusion rises in percent value as the cuff pressure decreases. The physician conducting the skin perfusion test will likely record the value of skin perfusion pressure as forty-five millimeters of mercury. FIG. 2B again illustrates a display from a prior art monitor with the perfusion measurement rising in percent value as the cuff pressure decreases. However, motion artifact is now displayed at forty-five millimeters of mercury. A physician conducting the skin perfusion test may erroneously record the value of skin perfusion pressure as forty-five millimeters of mercury.
A further problem with conventional devices is that reproducibility is inhibited because when repeated measurements are needed the laser Doppler optical probe is not necessarily placed at the same site. Consequently, the surface perfusion pressure measurements may vary because the sensor is typically placed on a different site having different microcirculatory flow. For example, fiber optic probes may be placed directly on the surface of a patient's tissue underneath the pressure cuff. If repeated measurements are necessary, the fiber optic probe or sensor may not be placed on the same site in subsequent measurements.
In addition, if the fiber optic probe is used on several patients, this can create the risk of nosocomial infections, and other infections that originate in hospitals or healthcare settings. The problem with infection is sometimes dealt with by using disposable probes or sensors. However, disposable probes are more expensive than non-disposable or reusable probes and can also be time consuming to remove and replace. Routine removal and replacement of probes can also create equipment errors, calibration problems and overall system malfunctions and perhaps more importantly can affect reproducibility.
Another problem that exists with conventional systems is that the laser Doppler probe or sensor is sometimes placed underneath the pressure cuff or distal to the pressure cuff for measurement. Because the laser Doppler sensor is measuring the transmission of light, it would be ideal to provide for a device that is useful in eliminating ambient light from the measurement site.
Thus, there exists a need for a device that can be used in conjunction with probes that does not require disconnecting the probe from a monitoring system in order to replace it with a new probe and that can address the problem of reproducibility and ambient light.
Given the foregoing problems with conventional systems, there is a need for a system that (i) eliminates the need to disconnect a probe from surface perfusion pressure instrumentation to measure an alternate site or take a measurement on another patient; (ii) provides for a sensor placement device that can assure reproducibility where repeated measurements are required and reduces ambient light; and (iii) compensates for, or eliminates, motion-induced artifacts in patient-attached critical care monitoring instruments. In addition, there is a need for a system that increases reliability and reproducibility by eliminating user-created error for example, variable inflation and deflation and/or variable interpretation of the results. A new and improved skin perfusion system that includes a sensor placement device and that automatically inflates and deflates the pressure cuff, controls inflation and deflation, and detects and rejects motion artifact, and automatically determines an SPP value is needed.